Various techniques are known for molding articles of different kinds of heat-hardenable, substantially liquid resins, particularly addition type polyester resins of the thermosetting character. See, for example, U.S. Pat. No. 3,216,060, incorporated herein by reference, and U.S. Pat. Nos. 2,962,764 and 2,962,767. The invention is an improvement over the methods described in the foregoing patents.
Throughout this specification and in the claims, the terms "upper" and "lower" are used in reference to mold parts of a compression mold by which the processes of the prior art and of the invention are performed. Generally, molds are comprised of one mold part which is atop another mold part. However, in certain environments, the mold parts might be side by side. In that case the upper mold part described herein is one of the two mold parts and the lower mold part described herein is the other mold part.
In conventional compression molding, the resinous composition is placed between the mold parts when they are in a fully open position. The mold parts are then moved together from a fully open to a fully closed, compressed condition during a period of approximately five to eight seconds. The now closed mold with the compressed resinous composition therein is heated so as to completely polymerize the resinous composition.
In the molding of liquid addition type resins, it is known to fill the cavities in at least one of the upper and lower parts of a mold with measured amounts of resin and to then place a sheet of an appropriate covering material, such as a material comprised of polyvinyl alcohol, over the liquid resin on the mold parts and then to bring the mold parts together to heat-set the material. In particular, in the disclosure of U.S. Pat. No. 3,216,060, the liquid resin is captured between two layers or sheets of the covering sheet material. Each layer is contacted by the respective one of the upper and lower mold parts when they are brought together. The sheets capture the resin material and prevent it from escaping from the mold, enable release of the heat-set resin from the mold after compression molding, and do not interfere with the compression molding.
Pearlescent material is frequently added to the thermosetting resins which are compression molded in order to give the resulting molded product a pearlescent, semi-glossy appearance. Various pearlescent materials are known, including fish scales. The pearlescent material is frequently in the form of flakes that are mixed with the liquid resin.
It has been found that pearlescent particulate materials initially uniformly mixed with a liquid thermosetting resin generally migrate to the outer surface of the mold part, to be nearer where the mold parts come into contact. Because of this, in button manufacture, this has led to the molds only being sub-cavity molds with a flat face side on one mold part facing a sub-cavity in the other mold part in order that satisfactory pearlescent appearance of the surface layer, often called the pearl orientation, may be obtained on the outwardly facing side of the button.
In compression molding using liquid thermosetting resins incorporating pearlescent materials, a number of goals are sought to be achieved. When a button or the like item is compression molded, it is desirable to have uniform pearlescence across the outwardly facing surface. This is more easily achieved when a button is formed with a flat front surface. However, as is frequently done with buttons, after removal of the molded piece from the mold and after separation of the molded piece from the layers of polyvinyl alcohol sheet material or the like sheet material, where such separating layers are used, the face or front side of the molded item is depressed, usually by grinding or cutting after manufacture. Uniform distribution of pearlescence across the fronts of the buttons, or the like molded pieces, even after they have been ground or cut, is desired.
The known technique of compression molding pearlescent button blanks has a number of drawbacks which manufacturers in this field have come to tolerate. The depth orientation of the pearlescence on the top side of the molded piece is a relatively small fraction of the total thickness of the piece, usually not exceeding about 121/2% of the thickness dimension. This means that although the pearlescent material is distributed throughout the thickness of the compression molded blank, there is a concentrated layer of pearlescent material which is relatively thin at the top of the molded piece. During the post-molding fabrication steps, such as machining, grinding or otherwise forming depressions in the molded piece or beveling its edges, the layer of more concentrated pearlescent material may be removed. The thinness of the surface layer, often called the orientation of pearlescent material, places substantial limits on the cutting or shaping of the molded piece as the orientation and pearlescent luster disappears completely when a cut is deeper than about 121/2 percent of the thickness of the piece. As a result, in conventionally molded pearlescent buttons that have been cut or milled, the cut or milled area of the button does not have the same luster as the uncut or not so deeply cut portions of the blank.
In addition, there is a tendency for some of the pearlescent material around the perphery of the cut blank to disappear, which creates undesirable translucent aread around the periphery. Were the pearlescent orientation deeper, this would not occur.
Furthermore, a compression mold for making a large number of buttons, or the like relatively small pieces, comprises a large number of cavities in which products are to be molded. Typically, although what has been calculated to be the correct amount of resinous composition has been applied between the mold parts, not all of the cavities in the mold parts are completely filled when the mold parts are compressed together, whereby many of the molded pieces must be discarded as incompletely formed and of poor quality. It is known already to charge the molds with excess resinous composition so as to properly fill more of the cavities. Yet, even with overcharging the mold with 35-45 percent more by weight of the resinous composition required to fill the cavities, still a large proportion of the available cavities are not completely filled, and many unusable pieces are therefore molded. The amount of overcharging required is dependent upon the size of the individual pieces being compression molded, with greater overcharging being required for larger size molded pieces.
Another aspect of the above described problem is that when buttons with shanks, or the like molded pieces with shanks, are conventionally compression molded, the shanks of many of the items do not fill out and are somewhat truncated. Especially when the button is attached to a garment at a hole through the shank of the button, the truncation prevents fabrication of usable buttons.
Further still, conventionally compression molded buttons have front surfaces that are slightly concavely depressed, which causes problems in fabrication or shaping, e.g. cutting of the button during shaping of the button.
Also, when relatively large molded pieces, larger than about 11/2" diameter, such as buckles or slides, are conventionally compression molded, they frequently warp.
The button blank or the like piece, which is conventionally compression molded, has a satisfactory pearlescent appearance on the top, which is preferably the front side of the button or piece. But, the rear or underside thereof has an unsatisfactory, substantially reduced pearlescence.
With conventional molding procecures, in order that the pearlescence may appear on the top or front surface of the molded piece, the molding procedure has been confined to sub-cavity molds, i.e., molds wherein the cavities are one of the two mold parts and, more conveniently, the upper mold part. It has been found that where button blanks are molded in compression molds with cooperating top and bottom mold cavities, a satisfactory degree of pearl orientation is not achieved in either the top or the bottom molds.